1. Field of the Invention
The present invention relates generally to an apparatus and process for applying thermoplastic adhesive and, more particularly, to an apparatus and process for applying the adhesive in a dot pattern to individual, flexible, die-cut parts.
2. Description of the Prior Art
Machines which apply thermoplastic adhesive to a moving web of material have been known in the art for many years. Typically, the basic process which such machines use in operation is to rotate an applicator roll within a tank of the melted thermoplastic adhesive. The adhesive picked up by the applicator roll is transferred to the moving web of material which is held against the applicator roll by a pressure roll. As a general rule, such machines also incorporate some type of wiper or doctor blade to remove excess adhesive and control the amount of adhesive on the applicator roll which is transferred to the web.
Over the years various problems have become apparent with respect to prior art devices. Specifically, controlling the amount of adhesive is often difficult. Significant hydraulic pressures are developed by the motion of the applicator roll and the resulting shear forces created when the adhesive on its surface is impeded by the doctor blade. This pressure build up can cause the wiper to be largely ineffective resulting in too much or uneven amounts of adhesive to be passed to the web. In addition, as adhesive passes under the wiper, minute quantities tend to stick to the top of the wiper edge. After a period of time, the adhesive accumulates to form droplets which may transfer back to the applicator roll and then onto the pressure roll, causing an excessive amount of adhesive to be applied in certain spots on the web and resulting in generally unsatisfactory performance.
Heating of the thermoplastic adhesive has also been a problem in the prior art. Prior to being melted, the adhesive is hard and is in the form of relatively large pieces, pellets, or powder. Obviously, it is undesirable for the particles of unmelted adhesive to come in contact with the applicator roll and interfere with the operation of the machine. Accordingly, prior art devices have generally provided for some type of arrangement for pre-melting the adhesive. This was often done by providing a separate tank in which the adhesive was melted, and providing a conduit to supply the melted adhesive to the tank containing the applicator roll. Other arrangements have been provided where the premelt portion of the structure is adjacent the actual holding tank for the melted adhesive. However, the arrangements of the prior art tend to be rather elaborate, resulting in additional expense associated with the construction of the adhesive applying machine. Furthermore, prior art devices have not made provision to isolate the heating elements from the body of the tank. This results in excessive heat transfer to the machine elements and frame, which causes carbonization of adhesive and greater energy use. Finally, the machines have been generally directed to the application of a single type of thermoplastic adhesive and have not been constructed in such a manner which allows easy cleaning of the machine and changeover to application of another type of adhesive.
As noted previously, machines of the prior art have generally been used to apply thermoplastic adhesive to a moving web. However, in recent years a need has developed for machinery which will apply the thermoplastic adhesive to individual, flexible, die-cut parts, such as shoe parts. Although there have been some attempts to adapt the known types of adhesive applying machines to this type of use, such efforts have not been successful. Die-cut parts vary in size, shape, thickness, material, etc. A machine designed to apply adhesive to a uniform web is simply not suited to apply adhesive to die-cut parts. There must be the capability for the machine to adjust to the variables introduced by die-cut parts. In addition, it is necessary for the parts of a shoe, for example, to remain flexible after the adhesive is applied. The application of a uniform coating of thermoplastic adhesive as known in the prior art greatly reduces the flexibility of the part to which it is applied.
The more recent prior art teaches that a process whereby the thermoplastic adhesive is applied in a selected pattern solves some of the problems associated with application of thermoplastic adhesive to die-cut parts. Specifically, the pattern application allows the part to remain flexible since the adhesive is in discrete quantities rather than a uniform layer. In addition, less adhesive is used in a given area, while retaining sufficient application to obtain the necessary strength for the adhesive bond. Use of less adhesive also means less heat transferred to smaller parts which enables the parts to be handled immediately after the application of the adhesive.
There is also a tendency of die-cut parts to stick to the applicator roll when the adhesive is being transferred to the material. Accordingly, the prior art shows the provision of pick-off fingers to guide the part away from the applicator roll. In such instances, the applicator roll is generally provided with grooves in which the pick-off fingers ride, to allow for an effective means of contacting and guiding the part away. This has resulted in a secondary problem wherein adhesive accumulates in the grooves of the applicator roll and is transferred to the pick-off fingers, requiring additional wiping means for the grooves in the applicator roll.
Accordingly, the teachings of the prior art do not provide solutions to the problems related to build-up of hydraulic pressures at the wiper, transfer of droplets of adhesive from the wiper to the applicator roll to the pressure roll, excessive heat transfer to the main frame, cleaning/changing adhesive, adjustments for die-cut parts and accumulation of adhesive on pick-up fingers; all of which are solved by the present invention.